feature request: improve annotations addition/edition/toggle in mobile

[abubelinha]

Based in the example annotations demo provided in #84, I think annotations are read-only in mobile browsers.

• They can be edited/added, but only in theory: in practice, I can't finally save user modifications into annotations js object: see below (so I can't even think about sending them to server-storage, which is a different subject).
• And I can't figure out how to toggle them on/off.

I couldn't finish an annotation in any mobile/touch interface I tried (3 different Android -all Samsung- and an old ipad Air ME898TY/A, iOS 12.4.6). Always same problem:

I can draw the annotation frame and fill in the three input boxes using "next" key to navigate till the last box; but there "next" dissapears and I cannot reach the Ok/Cancel/Delete buttons (so I cannot submit the form to php server).
When I try to press one of them with my finger, iipmooviewer behaves like if I was touching the underlying zoomable image.

This issue is important if we want to promote iipmoviewer annotations in contexts where most of the people will be using mobile browsers (i.e. class assignments for students, some citizen science collaboration projects, etc)

Just in case any of these can help:

• https://stackoverflow.com/questions/50581836/html-button-not-working-in-mobile-device
• Annotorious works great in mobile regarding buttons. It also supports polygons!! (see #51)
  I think it was designed for <img> elements, but it might work with iipmooviewer zoomable images. See:
  • Annotorious plugin for OpenSeadragon viewer.
  • Annotorious + Allmaps (IIIF images).
• Annotatorjs (I think this is mostly centered in text annotation, but it also has some other plugins).

Thanks
@abubelinha